An observatory of strings and light
Four kilometers from the shore of Lake Baikal, an array of sensors hangs suspended in the deep, cold water. This is the Baikal Gigaton Volume Detector (Baikal-GVD), one of the largest neutrino telescopes on Earth. It is not a telescope in the conventional sense; it uses nearly a cubic kilometer of water as its detection medium. The detector is a modular structure composed of independent groups called clusters. As of early 2026, the observatory consists of 16 clusters, with almost 4900 optical modules in total.
Each cluster contains eight vertical strings, each equipped with 36 spherical glass optical modules. These strings descend to depths between 750 and 1350 meters, where the pressure is immense and sunlight cannot penetrate. The optical modules, each containing a highly sensitive photomultiplier tube, are spaced 15 meters apart vertically. Construction and maintenance happen during the two-and-a-half months in late winter when the lake is covered by a thick layer of ice, providing a stable platform for deploying the delicate instruments. This unique construction method is one reason Lake Baikal was chosen, in addition to its extreme depth of 1,642 meters and remarkable water clarity.
Listening for cosmic whispers
The Baikal-GVD is designed to detect high-energy astrophysical neutrinos—subatomic particles because they rarely interact with other matter. These particles are born from some of the most violent events in the universe, such as exploding stars or the activity around supermassive black holes. Traveling unimpeded across billions of light-years, they carry information from their sources. When a neutrino happens to strike a nucleus in a water molecule, it produces secondary charged particles. These particles, moving faster than light in water, create a faint cone of blue light called Cherenkov radiation.
The array of optical modules detects this brief flash of light. By analyzing the timing and intensity of the light hitting different sensors, scientists can reconstruct the direction and energy of the original neutrino. The telescope is part of a Global Neutrino Network, working alongside the IceCube observatory at the South Pole and KM3NeT in the Mediterranean Sea to observe the entire sky. While IceCube observes the Northern Sky, Baikal-GVD's location makes it perfectly suited for monitoring the Southern Sky, watching for neutrinos that have traveled all the way through the Earth. Based on data collected between 2018 and 2022, the Baikal-GVD collaboration has confirmed the detection of a cosmic neutrino flux consistent with observations made by IceCube.